Retroviral vectors are excellent tools for the stable delivery of gene products into cells of therapeutic interest. However, use of these vectors in a clinical setting has recently been shown to be problematic. In a gene therapy trial for the correction of Severe Combined Immune Deficiency (SCID), two patients, out of eleven, developed T-cell leukemia as a direct result of an integrated retroviral vector. In an effort to prevent undesirable outcomes in future gene therapy trials, a primary goal of this study is the development and testing of novel vectors and strategies aimed at increased safety without the loss of effectiveness. New vectors will be developed that are less likely to lead to insertional mutagenesis while still retaining the therapeutic benefits. This will require the engineering of DMA elements with enhancer-blocking activity to inhibit transactivation of adjacent genes in genomic DMA. Incorporation of a regulatable suicide gene, which could be activated in the case of an adverse outcome, will provide an additional safety feature. Lastly, the vectors and strategies discussed above along with the use of limited numbers of genetically modified cells will be tested for their ability to rescue a genetic defect in a murine model of immunodeficiency. This project is aimed at tempering efficacy with safety in producing the safest, most potent retroviral vectors for use in human gene therapy trials.